The present invention relates to a brake booster device, and more particularly relates to such a brake booster device for a vehicle such as an automobile, particularly adapted to provide good braking action, especially during the initial stage of braking operation.
The present invention has been described in Japanese Patent Application Ser. No. Showa 60-296658 (1985), filed by an applicant the same as the applicant or the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
In the prior art, there have been proposed various types of brake booster device for supplementing the piston pressure of a brake master cylinder device, in order to reduce the actuating pressure that is required for the driver of the vehicle to exert on the brake pedal thereof. In Japanese Patent Laying Open Publication Ser. No. 56-13250 (1981), for example, which it is not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, there is disclosed such a brake booster device.
With such a brake booster device, when it is a hydraulic type of brake booster device as opposed to a vacuum type brake booster device, typically there is provided a control valve which is operated by depression of the brake pedal of the vehicle, and which when thus operated supplies pressurized hydraulic fluid to the actuating cylinder of a power piston. Thus, when operated, this power piston pushes the piston of the brake master cylinder device, or the primary piston of the brake master cylinder device in the case that said brake master cylinder device is of a tandem type, in the direction for squirting out brake fluid into the braking system of the vehicle.
There is however a problem with such a brake booster device, when as specified above the brake master cylinder device to which it is fitted is of a tandem type, in that only the primary piston of such a tandem type brake master cylinder device can be directly pushed by the brake booster device. Accordingly, since in a hydraulic type brake booster device the sliding resistance is inevitably substantially greater than in a vacuum type brake booster device, particularly in the initial stage of brake operation in which the brakes of the vehicle are first being applied, the supplementary force provided by the brake booster device for actuating the tandem type brake master cylinder device is relatively small. The consequence of this is that it is difficult for such a hydraulic type brake booster device, when fitted to such a tandem type brake master cylinder device, to provide as good a braking feeling as would a vacuum type brake booster device.